Orientations of planar cataclasite zones in the Chicxulub peak ring as a ground truth for peak ring formation models

1Naoma McCall,1,2Sean P.S.Gulick,3Brendon Hall,4.5Auriol S.P.Rae,4Michael H.Poelchau,6Ulrich Riller,7Johanna Lofi,8Joanna V.Morgan
Earth and Planetary Science Letters 576, 117236 Link to Article [https://doi.org/10.1016/j.epsl.2021.117236]
1University of Texas at Austin, Jackson School of Geosciences, Institute for Geophysics & Department of Geological Sciences, J.J. Pickle Research Campus, Austin, TX 78758, USA
2Center for Planetary Systems Habitability, University of Texas at Austin, Austin, TX, USA
3Enthought, Inc., Austin, TX, USA
4Institute of Earth and Environmental Sciences—Geology, Albert-Ludwigs Universität Freiburg, Albertstrasse 23b, Freiburg 79110, Germany
5Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
6Institut für Geologie, Universität Hamburg, Bundesstrasse 55, Hamburg, 20146, Germany
7Géosciences Montpellier, Université de Montpellier, CNRS, France
8Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK
Copyright Elsevier

Hypervelocity impact cratering is an important geologic process but the rarity of large terrestrial impact craters on Earth and the limited technical options to study cratering processes in the laboratory hinders our understanding of large-scale impact processes. Drill core recovered from the peak ring of the Chicxulub impact crater during International Ocean Discovery Program (IODP)/International Continental scientific Drilling Program (ICDP) Expedition 364 provides an opportunity to examine target rock deformation and thus, to assess cratering models in this regard. Using oriented computer tomography (CT) scans and line scan images of the core, we present the orientations of mm-to-cm-scale planar cataclasite and ultracataclasite zones in the deformed granitoid target rock of the peak ring. In the upper 470 m of the target rock, the cataclasite zones dip towards the crater center, whereas the dip directions for the ultracataclasite zones are approximately tangential to the peak ring. These two orientations are consistent with deformation expected from hydrocode-modeled principal stress directions for the outward movement of rocks as the transient crater develops, and the inward movement of rocks associated with collapse of the transient crater. Near the base of the core is a 96 m-thick interval of highly-deformed target rock with impact melt rock and rock fragments not observed elsewhere in the core; this interval has previously been interpreted as an imbricate thrust zone within the peak ring. The cataclasite zones below this thrust zone have different orientations than those in the 470 m-thick block above. This observation implies a differential rotation from the overlying block during the final stages of peak-ring formation. Our results support an acoustic fluidization process, wherein blocks that vibrate or slide relative to each other allow the target rock to flow during transient crater collapse, and that the size of coherent rock blocks increases over the course of crater modification as the target rock regains its cohesive strength and acoustic fluidization decreases.

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